The present invention relates to a new and improved device for monitoring the travel of thread-like objects, such as yarns, threads, wires or endless webs, e.g. in textile machines of various kinds.
Monitoring or sensing devices of the type using electromagnetic or piezoelectric transducers having a feeler member in frictional contact with a travelling yarn are well known in the art. Those known sensing devices may generate yarn travel signals of high amplitude; however, they are also responsive to shocks, mechanical vibrations, ambient noise and other trouble generally present and unavoidable in texitile plants. Other sensing devices or units for determining a relative movement between two bodies, e.g. a yarn and an insulated friction body, are described in U.S. Pat. NO. 3,676.769. These sensing units make use of electrical charges or voltages which are generated or induced in the unit by the travelling yarn, and thus they are hardly susceptible to shock, mechanical vibration and noise. The monitoring devices of the present invention constitute improvements of those disclosed in the aforementioned U.S. Pat. No. 3,676,769.
All of the mentioned known sensing or monitoring devices produce yarn travel signals in the shape of A.C.-waveforms. However, it frequently happens that the yarn travel signal is not really continuous and exhibits short interruptions even when the yarn is travelling. One cause of such interruptions may be transversal movements or vibrations of the yarn giving rise to changes in the friction between the yarn and the member of the sensing device which is in frictional contact with the yarn. Such interruptions simulate yarn breakages and may cause the textile machine to stop. It is evident that such unwanted machine downtimes must be avoided, since only a real yarn breakage has to stop the machine.